September 18, 2012

Black Holes Reveal Their Origins Through Dying Tones

Recent research from the University of Cardiff has found that the dying tones of black holes reveal the cosmic crash that caused them to form.

Black holes are regions of space where gravity is so strong that not even light can escape. Usually, they are the evolutionary ending of stars that were 10 to 15 times larger than our sun. After these stars undergo a supernova event, sometimes the remnant will collapse in on itself, creating a singularity. Because the gravity is so strong that light can't escape, no information comes to our scientists from the surface area, called an event horizon. Isolated dark holes don't even emit any form of radiation.

Sometimes, though, black holes are deformed through other black holes or stars crashing into them. These deformed black holes are known to emit a new sort of radiation, called gravitational waves, which were predicted by Einstein nearly one hundred years ago.

Gravitational waves are ripples in the fabric of spacetime that travel at the speed of light, but they are extremely difficult to detect.

Europe, Japan, India and the US are building kilometer sized laser interferometers to detect these waves from colliding black holes and other cosmic events. The interferometers are sensitive to gravitational waves in roughly the same frequency range as audible sound waves, and can be thought of as a microphone to gravitational waves.

When two black holes orbit around each other, they emit gravitational waves and lose energy. Eventually, they collide to produce a black hole that is initially highly deformed with gravitational waves that come out not in one tone but in a mixture of different tones, very much like the dying tones of a ringing bell.

The frequency of the tones from a black hole depend on only two factors: the mass of the black hole and how rapidly it spins. Therefore, scientists have long believed that by detecting the spacetime ripples of a black hole and measuring their frequencies one can measure the mass and spin of a black hole without going anywhere near it.

The research team used Cardiff's powerful ARCCA cluster to perform a large number of computer simulations of a pair of black holes crashing against each other. The results, published in Physical Review Letters, show that the different tones of a ringing black hole can actually tell us much more.

"By comparing the strengths of the different tones, it is possible not only to learn about the final black hole, but also the properties of the original two black holes that took part in the collision," explained Ioannis Kamaretsos, who performed the simulations as part of his PhD research.

"It is exciting that the details of the late inspiral and merger are imprinted on the waves from the deformed final black hole. If General Relativity is correct, we may be able to make clear how very massive black holes in the centers of galaxies have shaped galactic evolution. We never guessed it would be possible to weigh two black holes after they've collided and merged," said Dr Mark Hannam. "We might even be able to use these results to test Einstein's general theory of relativity. We can compare the waves we observe from the orbiting black holes with the waves from the merged black hole, and check whether they are consistent."

Professor B Sathyaprakash, who has spent his whole career studying gravitational waves commented: "It is quite remarkable. As in any new research, our finding opens up new questions: how accurately can we measure the parameters of the progenitor binary, whether our results hold good for more generic systems where initial black hole spins are arbitrarily oriented, etc. We will be addressing these questions in the coming years. Advanced gravitational wave detectors that are currently being built will provide us an opportunity to test our predictions in the coming decade."

The team's research opens up a new avenue of approach for studying the properties of the binary that produced the final black hole even when the binary is not visible to a gravitational wave detector. Future gravitational wave detectors should be able to study black holes far heavier than what was thought possible before and hence enhance their science reach.